Redundant force summing servo unit

ABSTRACT

The servosystem which will tolerate failure in two of its four redundant channels and still be operational consists of four identical redundant units. The force outputs of the four redundant units are summed together, by means of a summing link or member, with the resultant force on the member being applied to a control valve of a fluid operated motor which is a low resistive load. The input signal to each redundant unit is an electrical signal which energizes a torque motor. The torque motor responds by developing a force on its armature, which force is transmitted to a jet pipe valve. The jet pipe is deflected thereby causing a fluid pressure differential at the receiving holes coacting with the jet pipe nozzle. This pressure differential is transmitted to both a mod piston (which is the drive piston of the redundant unit) and the pressure sensor piston in parallel with the mod piston. The motion of both pistons is force fed back to the jet pipe by means of yieldable, impositive members as flat springs, thus closing two independent loops.

i United States Patent TORQUE MOTOR [72] lnventors Robert F. Rasmussen3,270,623 9/1966 Garnjost et al. 91/387 Brooklyn Center; 3,272,0629/1966 Flippo et al 91/384 pp No gg gi Golden valley PrimaryExaminerPaul E. Maslousky Attorneys-Roger W. Jensen, Charles J. Ungemachand [22] Filed Nov. 17, 1967 Gordon Reed [45] Patented Jan. 12, 1971[73] Assignee Honeywell lnc.

MmneapolfsiMmn' ABSTRACT: The servosystem which will tolerate failure ina corporat'on of Delaware two of its four redundant channels and stillbe operational consists of four identical redundant units. The forceoutputs of the [54] REDUNDANT FORCE SUMMING SERVO UNIT four redundantunits are summed together, by means of a 11 Claims 6Drawin H S summinghnk or member, with the resultant force on the v g g member beingapplied to a control valve of a fluid operated [52] US. CL 91/3, motorwhich is a low resistive load ,9 3 The input signal to each redundantunit is an electrical [51] Int. Cl ..F15b 13/02, signal which energizesa, torque motor. The torque motor Fl5b 13/ 1 6 responds by developing aforce on its armature, which force is [50] Field of Search 91/3, an mittd to a jet ipe valve. The jet pipe is deflected y), 363A, y); therebycausing a fluid pressure differential at the receiving Y) y) holescoacting with the jet pipe nozzle. This pressure differential istransmitted to both a mod piston (which is the [56] References cueddrive piston of the redundant unit) and the pressure sensor UNITEDSTATES PATENTS piston in parallel with the mod piston. The motion ofboth 2,947,286 8/1960 Baltus et al. 91/387 pistons is force fed back tothe jet pipe by means of yieldable, ,171, 2 1965 Rasmussen 91/388impositive members as flat springs, thus closing two indepen- 3,190,1856/1965 Rasmussen 9l/363A dent loops.

"l7 PEDALS lo To PRE-FLIGHT -EQUIUZATION w MONITOR TEST esteem 9 r20 2si 1' si us A TRANSDUCER DEMO sHAPiNG VALVE r38 LAY AMP 23 25 mm MODPISTON ""JJ 7 I l I SAS 1 9 TRANSDUCER H ing SHAPIN6 EOUILIZATION r-DMONITOR D.C. a i SAS l '4 5 H l c DEMOD TORQUE VALVE AND TRANSDUCER AMP-DSHAPING MOTOR MOD PISTON SAS L T0 T0 AFT n TRANSDUCER DEMOD asHAPINGEOUILIZATION MONI'IOR Roma AMP I QC. 8 ILONGITUD.

l'iz--1 4 1 I POWER ACTUATOR I TORQUE .VALVE AND MOTOR MOD PISTON 1 L iT0 EOUILIZATION b MONITOR I D.C.8 ."2---- 5.0.

MOD PISTON .VALVE AND 4 I I PATENTEUJANIIZIBYI i 3;'554;O8.4

,SHEET 2 0P4 SOLENOID EmAGE FEEDBACK SPRING MOD PISTON f summus LINK 38FIG. 2

FEEDBACK 64 seams r52 6' VOLT TORQUE JET MOD .L n

MOTOR PIPE PISTON s 63 Egg; FILTER I T 52 spams PRESSURE Y :3 s SENSORLIMIT DISENGAGE senvo L FILTER swncn mu NOTIFY PILOT 1mi INVENTORSROBERT E RASMUSSEN JOHN C. TAYLOR ATTORNEY SOLENOID P ENGAGE VALVECHANNEL A COMMANDS PATENTEUJANIEIBYI 554 SHEET 3 OF 4 ENGAGE 4 gggn ggOPEN ENGAGE CIRCUIT IF U 72 c,+c .SPS 50 f c c a PREssuRE l 2 ';s

SENSING SWITCHES 2 s I 1} WHERE c, a c ARE CYLINDER 69 PREssuREs a HFEEDBACK PS IS SUPPLY PREssuRE SPRING I .E 67 I EQUALIZER r UE JET- PIPEVALVE s0 CHANNELA l 64 CYLINDE FEEDBACK SPRING CHANNEL 5 COMMANDSCYLINDER v OUTPUT CHANNEL c ,CHANNELC CWMANDS cYuNoER CHANNEL 0 CHANNELDCOMMANDS CYUNDER A INVENTORS ROBERT E RASMUSSEN JOHN C. TAYLOR ATTORNEYPATENTEUUANIZIB?! 3554.084

sum u' 0F 4 L++A2 PULSED FOR al |o4 PRESSURE MONITORING MART EN=G. ,7SWITCH 4 RESTRIC'TOR 2a 25 fi 4 Y EQUAL|ZER JET-PIP 59 FEEDBACK s amsSERVOVALVE s9 54 \cvunosn 52 5 FEEDBACK spam I. v. 4 l I Y I I FORCE Y vY suumme 1; LINK C J CHANNEL A CYLINDER \63 L J CHANNEL 8 CYLINDERCHANNEL-C CYLINDER CHANNEL D CYLINDER INVENTORS ROBEQT E RASMUSSEN JOHNC. TAYLOR BY ATTORNEY 'REDUNDAN' lf-FORCE' UMMING SERVO UNIT 1 Thepurpose of the pressure sensor piston is to provide long term hydraulicpressure equalization on all'of the jet pipe receiving holes.-Forexample, ideally if each of the four redundant units was identicala'ndr'eceived-the' same input signal,

the differential force on v eachmod piston because of the feeda back, inthe steady state, would be veryjlow. However, this ideal condition cannever be exactly attained because of differences during manufacturingresulting'from tolerances and driving signal mismatches so it ispossible'for steady state extraneous forcesito build up on the modpistons and summing link. The forcesare inbalance and dojn'ouseful'work. Some forces would be directed to the right,-and others tothe left, with-a net force on the summing-link being'zero.

Since a pressure sensor piston 'receivesfthe same pressure differentialas its respective mod piston, it is possible to-etfectively measure theforce on each mod piston by noting the displacement of its pressuresensor piston over a long period. The possibilityobviously exists thenof feeding back negative pressure sensor displacementto the jet pipe- Byso doing, thedifferential pressure forces on the mod pistons could berelieved by repositioning the jet pipes.

Each redund ant channel of the servo in one embodimentadditionallyincludesa third piston arrangement which acts as a failuremonitorflf a hardover faiiuretorra loss of input or feedback) occurredin any individualredundant channel, the pressure differential on bothits mod and sensor pistons would build up to a large value. Both-pistonswould move in response to the pressureHowever, the mod pistonisconstrained by the summinglink, which is beingheld in place (within asmall erfor) by the other redundant units; whereas the monitor pressuresensor piston is free to move, and does so, until it trips a failureswitch which disengages the particular redundant channel or unit as byremoving hydraulic pressure.

This invention relates to a novel fly-by-wire control apparatus"incorporating duplicate or redundant units. The term fly-by-wiremay be.visualized as a structural departure with respect to a conventionalaircraft having force transmitting control cables extending between thecontrol stick operated by the pilot and the-control surface or controlsurface actua I tor. ln-the fly-by-wire apparatus," such cables areomitted and signals are t'akendirectly from the pilots stick or othersignal sources for'operating a motor to position the actuator.

, coNvE rloNAL CONTROL SYSTEMS Existing conventional mechanical primarycontrol systems, wherein the pilotthrough his control wheel and cablesor linkage operates ae'ontrol surface-ofan aircraft or controls the mainactuator for the control surface, have reached a limit of performancedue to the basic operation limitation of mechanical elements insuch'system. For example, such primary conperformance and variablegeometry aircraft, sensitivity of the system to maintenance errors ismagnified in such system, and the safety and reliability of the systemdecreases.

Additionally, as the mechanical linkage system is basically a singlechannel system with considerable cross-sectional area from thearrangement of its operable elements, a-high vulnerability to combatdamage exists, Y

.REDUINDANT FLIGHT CONTROL LINKAGE v x v APPARATUS Y The abovedisadvantages of conventional mechanical control linkages in anaircraftcontr'olsurface positioning apparatus canbe eliminated by use ofelectrical and electronic techniques. Thus, anelectrohydraulic.mechanization of the fore, a redundant automatic flightcontrol'system can provide higher reliability and safety as well asimproved performance over the above type of primary control system.

One object of this invention therefore is to provide an improvedapparatus with channel redundancy and with a minimum requirement forcross channel monitoring.

Another object of this invention is to provide for channel I redundancymonitoring with a simplein-|ine" monitor concylinder configuration.

figuration eliminating requirements for cross channel comparisons andvoting logic.

A further object of this invention is'to provide redundant channels eachhaving a separate force summed servo which servos coact on acommon'output member to minimize movement of the output member uponoccurrence of a failure. v

A further object of the invention is to provide in each of multipleredundant channels a hydraulic or fluid pressure equalization monitorarrangement to minimize channel mistracking. I j I' A further object ofthe invention is to provide for each of the redundant channels anindividual improved malfunction equalization disengage monitorarrangement;

FIG. 3 is a block diagram of a redundant channel along with the summinglink or summing member for the four channels;

FIG. 4 is a block diagram of a second form of jet pipe equalizationpressure arrangement and monitor failure control; 1

FIG. 5 is a schematic diagram of the arrangement of FIG. 4; and

FIG. 6 is a schematic of a modified form 'of equalizer piston- In theapparatus, four identical channels, which receive like control signals,each include a force summed-fluid servo. The servos may have separatefluid sources or there may be a lesser number of fluid sources thanservoswith suitable pressure operated switching means for each sourcefor switching out a failed source from a servo and switching in anunfailed source to the servo, for example. The four servos are utilizedto drive a common output member with each servo driven directly fromoperation of a member of one of the redundant channels. Multiple forcesumming minimizes the output link response to a failure of one channelthus allowing a simple failure monitoring arrangement to disengage afailed channel.

Also a simple pressure equalization arrangement for a fluid servo isused in each channel to minimize the mistracking due to mismatching ofinputs or tolerances within the servo.

Response requirements of the servo failure monitor are not as criticalwith force summed servos (relative to displacement summed servos) asslowness or even failure of the monitor to disengage a hardover servo,for example results in only a small movement of the output link.

In each channel, a jet pipe valve utilizing an electrically drivenmoveable orifice is used to provide-proportional control and also tominimize hydraulic contamination effects which sometimes occur in otherarrangements.

A simple hydraulic pressure sensor piston monitors the differentialpressure across each individual servo power piston and feeds back a timeintegrated and limited signal to the torque motor output which reducesthe servo differential pressure and thereby channel mistracking. Theprimary positional feedback to the torque motor linkage that positionsthe jet pipe is obtained from the common multiple force summing memberthat is positioned by the four redundant servos.

Referring to FIG. I, a redundant control system includes a plurality ofredundant channels A, B, C, and D. Since the channels are similar, adescription of one will suffice for a description of all. Thus referringto channel A, an electrical variable signal source such as controltransducer 17 operated by movement of a member such as the control stickof an aircraft supplies the control signal in each case to a demodulatoramplifier 19 which in turn through a shaping network 20 supplies acontrol signal to summing device 21. The summing device also receivescontrol signals from stabilization and control augmentation systems (notshown) through conductor 22. The signal from summing device 21 istransmitted by conductor 23 to a servoamplifier 25. The output of theamplifier 25 reversibly controls the operation of a conventional torquemotor 26 which through motion transmission means 27 controls theoperation of a moveable servo control member that differentially portsfluid to the ends of a piston of a servo 38. The piston under control ofthe fluid applied thereto operates through piston rod 61 a common memberof the four channels, and the common member 40 has an output 41 thatoperates for example the control valve of a main actuator for a controlsurface.

The movement of the servopiston and its rod 61 to force summing member40 is also supplied in a feedback arrangement of the force transmissiontype to a force summing arrangement within servo 38. The output from theforce summing arrangement within servo 38 is transmitted to the moveableservocontrol member.

Additionally, the force summing arrangement within servo 38 receives aninput over transmission means 48, of the force type, from a pressureequalization arrangement 49.

The equalization arrangement 49 and a monitor 51 receives as inputsthereto the pressure across the mod piston in servo arrangement 38 to bedescribed.

FIG. 2 shows a force summed fluid-type servomotor 38 for one channel.Servo 38 includes a torque motor 26 (that receives the variabletransducer signal) having an output arm 54 which through a link 59operates to variably displace a jet pipe or moveable servocontrol member52. The jet pipe 52 receives pressure fluid through a solenoid operatedengage valve and conduit 51 from a fluid pressure source P. A return 53is provided from the discharge of jet pipe 52. The jet pipe 52 coactswith two holes or ports 57, 58 which normally receive equal fluidpressure discharge from the jet pipe 52. The holes 57, 58 communicate bysuitable conduits to opposed sides of a mod piston 60. Operation of thetorque motor arm 54 and its connected link 59 in either directionaccording to the transducer signal displaces the jet pipe 52 to causereversible movement of the mod piston 60. An output rod 61 of the modpiston 60 connects to a mod piston summing link or summing member 63which may be used to position a control valve of a main actuator, nowshown. The jet pipe 52 is repositioned toward its normal position by afeedback arrangement comprising summing member 63 on common member 40, afeedback spring 64 and link 65 connected to torque motor arm 54. Theredundant servos operated member 63 thus exerts supervision of thefeedback to jet pipe 52.

A hydraulic fluid cylinder-piston type monitor having a piston 67 hasapplied to opposite ends thereof pressure. The fluid is conducted to theends through suitable orifices 62 or restrictors in subconduitsconnected with main conduits extending to opposite sides of the modpiston 60. The orifices provide a time delay to operation of the piston67 following initial displacement of mod piston 60. Movement of themonitor piston 67 is transmitted through a feedback spring 69 (thusthrnnoh a force or imoositive feedback member) to the torque motor arm54 to reposition jet pipe 52 to thereby provide pressure equalizationfromjet pipe 52 on the holes or ports 57. 58.

In addition, although a separate monitor may be provided. the hydraulicfluid clinder-piston monitor having piston 67 operates through asuitable connection a disengage limit switch 72 for opening theelectrical circuit through a solenoid operated engage valve 50permitting the valve to close by suitable means such as a spring tointerrupt the flow offluid to the adjustable jet pipe 52, thusindicating a failure in the particular channel involved.

FIG. 3 is an analysis block diagram of the force summed servo 38 of FIG.2 wherein the electrical torque motor 26 receives a variable voltagesignal over a conductor from amplifier 25 FIG. 1. The torque motor as atransducer converts the electrical input to a mechanical torque outputproportional to its electrical input. Through its arm 54 and connectinglink 59, it initially positions the jet pipe 52 a distance proportionalto the difference between the torque motor output and the two feedbackspring (64 and 69) forces.

Due to its displacement, the jet pipe 52 varies the fluid pressure inreceiving holes 57 and 58, proportional to its displacement, whereby themod piston 60 is positioned in inches per second in accordance with theflow resulting from that pressure differential which is applied to itsopposite sides. The piston 60 moves in its cylinder in accordance withthe integral of the time period of the differential pressure andresulting rate of flow, and through its rod 61 positions the mod pistonsumming link 63 and member 40 which operates the feedback spring 64 toprovide a force feedback proportional to'thc displacement of the summinglink 63 from the neutral position. The force on spring 64 varies thenormal dimension or shortest distance between its output-input points.This force is transmitted through the connecting link 65 to the forcesumming point 66 which may be considered the remote end of arm 54.

While the servoloop has been described, there is also included a secondloop for repositioning the jet pipe 52 through the pressure sensormonitor piston 67. The difierential pressure on ends of the mod piston60 resulting from any load on said mod piston because of disagreementswith other mod pistons connected to summing link 40 is also appliedthrough the suitable filters or orifices to the pressure sensor piston67 which has a total displacement or movement dependent upon the timeintegral of the differential pressure supplied to the opposite sides ofpiston 67. This displacement is converted through the spring member 69to pounds per inch of displacement of piston 67 and is also applied tothe force summing point 66.

Additionally, in the embodiment of FIG. 2 the displacement of pressuresensor monitor piston 67 is supplied to the disengage on-off limitswitch 72 which operates as will be described hereinafter to break thecircuit through the operating solenoid of engage valve 50 therebyterminating the flow of fluid from the pressure source P to the jet pipe52. The switch 72 also controls the energization of an indicator fornotification to the pilot of such failure in the channel.

As indicated in FIG. 3, X2, X3, X4, the mod pistons of the three otherredundant, force servos are connected to the summing member 40. It willbe understood that each channel torque has similar feedback provisionsas that in channel A.

While mechanical feedback has been described specifically to applyimpositive torques to the jet pipe 52, it is contemplated that thefeedbacks from summing member 40 and pressure sensor piston 67 could beelectrical signals that would, instead of being applied to torque arm 54as the mechanical feedback are, be applied to amplifier 25.

Concerning a second embodiment, and advancing from FIG. 3 thearrangement in FIG. 4 resembles FIG. 3 except for the fact that thedifferential pressure applied to the mod piston 60 is not only appliedto the mod piston pressure equalizer 67 but is separately applied to theengage valve control circuit, switch 72 having a separate pressureresponsive piston operating means. FIG. 4 thus hasreference characterscorresponding to those in FIG. 3. I

In addition to the mod piston pressuresensing or responsive switches 72controlling the solenoid engage valve 50, the valve may additionally becontrolled from engageand monitor-circuits to be described. FIG. 4consequentlyincludes logic statements which will hereinafter bereviewed.

FIG. 5 shows the force summed servo'with the mechanical feedback of FIG.4shown indetail. The features common with FIG. 2 havethe same referencecharacters.

FIG. 5, since it presentsmetely. novel electrical circuits over materialpreviously'considered, will be described concurrently with itsoperation, thus momentary closing of a manually operable start monitorswitch 80 upper Ieft'closes the'circuit from a DC voltage source (notshown), conductor 81, switch 80, conductor 82, conductor 83,: presentlyclosed solenoid operated switch 84, conductor 85, through a pull-inelectrical winding 86 of a push-type solenoid, conductor 87 to DCreturn. Previous to operation of switch 80, the engage valve 50 as shownis in closed. position whereby pressureifrom the source P cannot betransmitted to the jet pipe 52. Following closing of switch 80 andenergization of winding 86, the valve moves rightward in the'FlG. thuspermitting pressure fluid to be supplied to the jet pipe 52 andconcurrently opening the circuit through switch" 84 by an extensionof'the valve and valve 50 remains open by the following arrangement.

Pressure monitor disengage switch 72 as shown consists of fourrelatively moveable'end. mounted members twoof which, 90, 91 areelongated to engage with shoulders on moveable plungers 93, 94, of themod piston pressure monitor. Each of the separate plungers 93, 94 haveengaged therewith suitable outwardly biasing springs 95, 97. It isevident that when pressure fluid through suitable conduitsshown,isadmitted. to one end of each of the plungers 93 and 94 thatthenormalpressure in ports 57, 58 as transmitted by the conduits compress thebiasing springs 95, 97 causing the, operable inner switch members 90, 91to contact each other. The outer switch contacts 98, 99 which normallyengage their respective inner contacts 90, 91 remain so engaged.

Thus, as involving switcha72-and its'closedxcontacts, upon opening ofengage valve byoperationzof switch 80, a holding circuit for the engagevalve solenoid'is established from an electrical supply as a DC source(notshown), conductor 88 closed engage switch 104,0uter contact99,-contacts 91,90,

98, conductor 100, conductor-101', hold winding 102, pull-in winding 86,conductor87to DC return. I I j Concerning the operation ofthepressuremonitor disengage switch 72, in the event thatthere. has beena failed operation malfunction in signal operated-amplifier 25 ofchannel A for example whereas the remaining channels'have operated,motion is applied-in 'channelA through the force summing link 40 and rod61 to piston moving the cylinder feedbackflat spring 64 and displacingthe jet pipe 52, lwhich-has notorque applied thereto from torque 26, inaccordance with the dis spring 119. Similarly, the'chamber- 113'includes a washer '1 18' biased by a preloaded coil spring. 120. Thearrangement is placement of the output member 40 or summing linkt63.This unrestricteddisplacement ofpipe 52 results he large pressure in oneor the other of holes or ports 57, 58 which can approximate percent ofthe total pressure P, resulting in a large displacement say of plunger94 relativetoplunger 93 or viceversa and as to plunger 94 engagingswitcharrn thereby opening the electrical circuit between outer contact98 and inner contact 90 or as to plunger 93 beingoperated disengag ingouter contact 99 and inner COIIMCI-QLIIICI'CIJ}! opening the electricalcircuit between conductors" 88 and Ithus deener' gizing the windings102, 860i the solenoid causing the engage valve 50 to move to its closedpositionasshown in FIGS. The closing movement of the 'va lve50may beobtained by a suita-.

ble auxiliary spring means 89 connected thereto aided by the pressurefrom the hydraulic power source. As inFIG. 2, vin FIG. 5 the springbiased,.mod'piston pressure equalizer piston 67 is supplied'with fluidthrough'a 'rest rictor 62 to limit' the flow rate thereto sothatitoperateson a long term basis.,ln one mode of :operation ofthepressure' equalizer piston 67, in the event that the A channel mbdpiston 60 is jammed and will notmove, a differential pressu're on theopposite ends of the mod piston 60 for an extended time period occurs.The equalizer piston67, because of the presence over the time period ofthe differential pressure, is displaced and through its springconnection 69 will return the jet pipe '52 to normal position where itapplies equal pressure to the two openings 57, 58 and thus toboth endsof the mod piston.

Thecircuit between conductor 88 and outer contact 99 of limit switch 72as stated includes a manually operable engagedisengage switch 104 sothat the channel may be also manually disengaged. It is moved to closedposition following momentary operation. of switch 80 to hold thesolenoid valve 50in the open or engaged position. 1

With respect to the logic statements of FIG. 4, note that iii FIG. 5 thepressure line, to one end of monitor'pressure switc plunger 93 remotefrom its biasing means, has been labeled 2 and the line conveyingpressure to one end of p'lunger9,

remote from its biasing spring is labeledc Additionally the lineconveying the pressure from the source to the jet pipe 50 is labeledP,,. In. FIG. 4, the disengage logic is tabulated. In other words, theelectrical engage circuit for valve 50 is opened between contacts 90,91, if c, c be less than .5 P,. Also the circuit is opened betweencontacts 90,98 of the limit switch 72 if c, c, be greater than .8 P,Finally, if 0 c be greater than .8 P, the engage circuit is opened atcontacts 91, 99. In other words, for the first condition, the innercontacts 90, 91, will become disengaged or are not closed. For thesecond condition, outer contact 98 and inner contact 90 will bedisengaged. For the third condition or third logic statement, outercontact 99 and contact 91 will be disengageddn other words, the solenoidwinding for the engage valve 50 will be deenergized if there is very lowpressure supplied to the mod piston 60 by the jet pipe 52. Also, if thepressure be ex cessive on one side of the mod piston relative to theother. either switch contacts 98,. 90 or 99, 91 will be disengaged.

FIG. 6 shows a modified form of cylinder-piston equalizer 110, where thecylinder has a small diameter portion Ill Endchamber 112 includes awasher 1l7'which abuts one end of chamber 112 while subject to the forceof a prestressed coil such that the washers 1'17 and 118 with nopressure applied to conduits c, and c apply no force to the piston 67.However, when a differential pressure is applied to the conduits c,, c,the

pressure must overcome either one-of the preloaded springs 119, -beforeany movement is applied to the piston 67 for static load thereto. Bythearrangement of FIG. 6, there will be no action or-movement of theequalizer piston 67 until the static load exceeds the preloading ofeither one of the springs l 19, 120, in other words by use of thesprings 119, 120 and the arrangement of FIG. 6a dead spot'- is providedso' that operation of thepiston 67 does-not occur until the differentialpressure exceeds-the preloadingof either spring 119 or 120 thus staticloads are tolerated.

OPERATION Each of the servo amplifiers, such asamplifier 25in channel A,for the forcesummed fluid servos receive a similar electrical controlsignal. The multiple mod pistons are normally linked together to acommon output member 63. In accordance with the common control signals,the mod pistons normally or ideally have the same operation. In actualpractice, when such a mechanical arrangement is made, the null of eachof the servounits will vary in accordance with manufacturing tolerances,material variations, etc. To compensate for these effects and toequalize the load-carrying capacity of the individual mod pistons, adifferential pressure sensor equalizer is connected to the correspondingoutput cylinder to have applied thereto the differential pressure on amod piston. To permit only long term pressure variations to effectoperation of this mod piston pressure equalizer, flow to its oppositeends is limited by orifices. This limitation-plus the effect of thepressure sensor displacement characteristics permits the input signal tothe jet pipe to be modified such that the loads imposed on all the modpistons are equalized, or nearly so.

Should a failure occur in but one of the redundant units for example ahardover failure due to loss of servo feedback the pressure sensor(failure switch) of that unit because of high pressure due todisplacement of pipe 52 from center will move off towards one extreme.The opposing load on the remaining redundant mod pistons will be afraction of this pressure value since they oppose the failed channelcollectively. With the failure switch 72 set to 80 percent of theavailable pressure, P,, from the pressure source, only the failed modpiston or servo unit achieves this pressure level and, thereby, tripsits failure switch 72 indicating failure and disabling the fluid supplyto its jet pipe. The remaining channels continue engaged or operative.

This type of operation requires that the mod pistons handle little or noload as stated. This is accomplished by having the mod pistons drive astandard control valve for a main actuator. While there are some dynamicloads reflected to the mod pistons, the effect of the pressure sensor(equalizer) line orifices minimize these effects.

Either soft or hardover failures are detected directly by the failureswitch 72. Dead failures are detected due to the relativelyhigh-pressure gain utilized, which, when a command appears to the goodservos makes the dead servo appear to be hardover in the oppositedirection.

The four channels may have individual fluid sources and their jet'pipesmay be engaged and will remain engaged with at least one fluid sourcethrough pressure responsive switching in the event its original pressuresource fails.

It will be understood that the gains of the redundant channels are sodesigned that when the overall system calls for the operation of apressure equalization monitor, such as 67 FIG. 5, there is littleprobability of operation of a disengage switch 72, in response to suchcall.

FAILURE MONITORING The force summed servos with hydraulic mod pistonpressure equalization provide unique, straight-forward monitoring ofchannel performance. When channel mistracking reaches a predeterminedlevel, it is, by definition, a failure of that channel and a switch isactuated. Essentially, this method of monitoring compares each servoagainst the average output for all servos in that axis. Thus, nocomplicated logic arrangement is required to sort out the failedchannel. In the four channel embodiment herein, even after two channelsfail and are disengaged, the system is fail safe since if a malfunctionoccurs in one remaining channel, any tendency for irregular operation ofthe output member 40 is compensated by the remaining properly operatingchannel.

It will now be evident that there has been provided a novel controlsystem such as a fly-by-wire system" consisting of redundant channels,having fluid operated servos. Such systems, because of the redundancy,provide a fail operational arrangement more reliable than theconventional primary control system. Further, the arrangement includes aselfequalization arrangement for equalizing the fluid pressure on theholes cooperating with a jet pipe in each redundant channel.Additionally a disengage monitoring arrangement has been includedresponsive to fluid pressure on a servo to disable a failed servounit.

We claim:

In a fluid type actuator having a power output section and a controlsection, receiving fluid under pressure, for porting fluid throughsuitable passages to said power section, said control section have adisplaceable fluid conducting member and coacting receiving ports tovary the flow rate to the power section and thus the velocityofdisplacement thereof in combination:

means for variably displacing said member from a normal positionrelative to said two coacting fluid receiving ports;

yieldable feedback means between the member and power output sectionoperated from said power section to modify the position of or returnsaid member and ports to the normal position; and

further fluid pressure monitor means including a controller havingsubpassages, at least one having flow restrictive means, connected tosaid passages to said power section thereby responsive by saidrestrictive means to time duration of fluid pressures applied to saidpower section, operating on the member to control the rate of flow tothe power section.

2. The apparatus of claim 1 wherein operation of said controller in saidfurther means modifies the flow rate by modifying the position of saidmember to provide long term equalization offluid pressure in thepassages to the power section.

3. In a fluid-type actuator having a power output section and a controlsection for porting fluid from a source of pressure fluid to said powersection, said control section having a displaceable member to vary theflow rate to the power section and thus its displacement rate:

first means for variably displacing said member from a normal position;

second or feedback means operated from said power section to return saidmember to normal position;

a pressure fluid conduit means in said control section and having anoperable valve therein for transmitting fluid to said control section;and

pressure fluid monitor means connected to said power section thusresponsive to fluid pressure therein and controlling the operation ofsaid valve.

4. The apparatus of claim 3 characterized by the valve being of thesolenoid actuated type and the pressure fluid monitor means alsoincludes a switch means having contacts controlling a circuit of saidsolenoid and responsive to pressure to said power means to effectopening of said valve to supply fluid to the conduit.

5. The apparatus of claim 4, in including a pressure monitoring startswitch for momentarily energizing said solenoid to momentarily open thevalve to passage of fluid to apply pressure to said power means andswitch means.

6. The apparatus of claim 3, pluralized to provide at least threeredundant actuators, and wherein each power section thereof is connectedto a common force summing member, all power sections normally applyingeffects to an output of said summing member, whereby each feedback meansis operated in accordance with the average position of the output fromthe summing member whereby failure to cause a first means in one sectionto initially displace a displaceable member in the section results inthe feedback means repositioning said displaceable member in the sectioncausing a larger than normal operating pressure thus a change in nonnaloperating pressure to its power means with the accompanying operation ofthe pressure monitor means to terminate operation of said valve.

7. The apparatus of claim 6 wherein the first means displacing saidmember and the second feedback means thereof are structurally of theimpositive type.

8. The apparatus of claim 7 wherein the power section includes a powerpiston and wherein the displaceable member controls the position of anozzle or jet pipe which normally applies equal pressures to a 575?receiving ports or openings directly connected to opposite ends of thepower piston of the power means to apply normally equal pressuresthereto.

9. The apparatus of claim 7 including a pressure equalizer monitorresponsive to time duration offluid pressure applied to said powersection and operatively connected to said displaceable member. 7

10. The apparatus of claim 3, characterized by the valve being of thesolenoid actuated type andthe pressure fluid through their pistonsoperating a force summing common member connectable for operation tosaid output; first means displaceable for applying differential fluidpressure to said redundant servo motor pistons to effect ,operationthereof; second means responsive to excessive differential pressures ineach servomotor operating a failure switch connected to the first meansfor terminating application of fluid pressure to said servomotor thusterminating operation of said servomotor; and a differential pressureresponsive device in each channel connected through fluid flowrestrictive conducting means to opposite sides of its associated pistonto respond only to time duration differential pressures across saidpiston and connected to the first means to modify the displacementthereof.

2. The apparatus of claim 1 wherein operation of said controller in said further means modifies the flow rate by modifying the position of said member to provide long term equalization of fluid pressure in the passages to the power section.
 3. In a fluid-type actuator having a power output section and a control section for porting fluid from a source of pressure fluid to said power section, said control section having a displaceable member to vary the flow rate to the power section and thus its displacement rate: first means for variably displacing said member from a normal position; second or feedback means operated from said power section to return said member to normal position; a pressure fluid conduit means in said control section and having an operable valve therein for transmitting fluid to said control section; and pressure fluid monitor means connected to said power section thus responsive to fluid pressure therein and controlling the operation of said valve.
 4. The apparatus of claim 3 characterized by the valve being of the solenoid actuated type and the pressure fluid monitor means also includes a switch means having contacts controlling a circuit of said solenoid and responsive to pressure to said power means to effect opening of said valve to supply fluid to the conduit.
 5. The apparatus of claim 4, in including a pressure monitoring start switch for momentarily energizing said solenoid to momentarily open the valve to passage of fluid to apply pressure to said power means and switch means.
 6. The apparatus of claim 3, pluralized to provide at least three redundant actuators, and wherein each power section thereof is connected to a common force summing member, all power sections normally applying effects to an output of said summing member, whereby each feedback means is operated in accordance with the average position of the output from the summing member whereby failure to cause a first means in one section to initially displace a displaceable member in the section results in the feedback means repositioning said displaceable member in the section causing a larger than normal operating pressure thus a change in normal operating pressure to its power means with the accompanying operation of the pressure monitor means to terminate operation of said valve.
 7. The apparatus of claim 6 wherein the first means displacing said member and the second feedback means thereof are structurally of the impositive type.
 8. The apparatus of claim 7 wherein the power section includes a power piston and wherein the displaceable member controls the position of a nozzle or jet pipe which normally applies equal pressures to a pair of receiving ports or openings directly connected to opposite ends of the power piston of the power means to apply normally equal pressures thereto.
 9. The apparatus of claim 7, including a pressure equalizer monitor responsive to time duration of fluid pressure applied to said power section and operatively connected to said displaceable member.
 10. The apparatus of claim 3, characterized by the valve being of the solenoid actuated type and the pressure fluid monitor means also includes a switch means controlling a circuit of said solenoid and responsive to continued large displacements of said member from normal position opening said circuit to return the valve to closed position.
 11. In control apparatus, means for operating a low resistive load output comprising: a plurality of redundant channels each having a piston type proportional operated fluid servomotor, all servomotors through their pistons operating a force summing common member connectable for operation to said output; first means displaceable for applying differential fluid pressure to said redundant servo motor pistons to effect operation thereof; second means responsive to excessive differential pressures in each servomotor operating a failure switch connected to the first means for terminating application of fluid pressure to said servomotor thus terminating operation of said servomotor; and a differential pressure responsive device in each channel connected through fluid flow restrictive conducting means to opposite sides of its associated piston to respond only to time duration differential pressures across said piston and connected to the first means to modify the displacement thereof. 